Place the potatoes in a medium saucepan, add enough cold water to cover and bring to a boil. Add 1 tsp salt, reduce heat and simmer until tender, 10 to 15 minutes. Drain, return to the pan and mash with the milk, cheese and ¼ tsp each salt and pepper; fold in the scallions; cover to keep warm.

Meanwhile, beat the egg in a shallow bowl until foamy. In another shallow bowl, combine the chips, paprika and ¾ tsp salt. Dip the fish in the egg and then in the crushed chip mixture, pressing gently to help them adhere. Transfer the fish sticks to the prepared sheet and bake until light golden brown and opaque throughout, 8 to 10 minutes.

Stir together the tartar sauce and tomatoes. Serve with the fish and mashed potatoes.

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